Unify Maniphest + Differential comment styles

Summary:
I want to add comments to commits, and they should obviously share code with the
nearly-identical comments in Maniphest and Differential. Unify code/style as
much as possible.

This program made possible by a generous grant from D1513.

Test Plan:

• Looked at a bunch of different Differential and Maniphest comments; they

appeared to render identically to how they looked before.

• Tested some edge cases like anchors and "show details" on description edits

in Maniphest.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T904

Differential Revision: https://secure.phabricator.com/D1686